The present work reports on extensive plankton sampling surveys in a Scottish sea loch used by wild and farmed salmonids and describes the spatial and temporal distribution of planktonic larval sea lice and identifies factors which affect the larval distribution. The planktonic louse larvae recovered were predominantly Lepeophtheirus salmonis (Krøyer) and so the findings of this thesis refer primarily to this species of sea louse. Copepodids occurred in significantly greater densities at 0 m than at a depth of 5 m, whereas nauplii occurred in significantly greater densities at 5 m than at 0 m depth. Densities of caligid nauplii and L. salmonis copepodids recovered from the open-waters of Loch Shieldaig correlated significantly with the counts of gravid L. salmonis on farmed Atlantic salmon at the farm in Loch Shieldaig. Furthermore, the densities of L. salmonis copepodids were significantly correlated  with the estimated total numbers of gravid L. salmonis on all farmed Atlantic salmon in the Loch Torridon management area. Densities of L. salmonis copepodids did not correlate with estimated numbers of gravid L. salmonis on wild salmonids. Atlantic salmon farms were identified as important sources of L. salmonis larvae and these larvae can be transported several kilometres from the point of release. Husbandry louse control practices were indicated to be able to significantly reduce the densities of L. salmonis larvae in the water column. These findings support the principle of synchronised sea louse management at a hydrographic/management area level.